Polarized X-Ray Absorption Spectra of Bromine-Doped Polyacetylene; Evidence for Highly-Oriented Polybromine Ions

Abstract

Polarized extended X-ray absorption fine structure (EXAFS) and near-edge spectra are measured on the Br K-edge for streteh-oriented trans -(CHBr _y ) _x . A large difference is observed in both the EXAFS and near-edge regions between the spectra with the polarization vector E parallel and perpendicular to the axis of orientation. The Fourier transform results or polarized EXAFS demonstrate that polybromine ions are highly oriented along the fibril axis in the dopant concentration 0.015 < y < 0.036. Bromines are also shown to be covalently bonded with carbon atoms of the polymer backbone through a substitution reaction and dominate the radial distribution of bromine in the direction perpendicular to the fibril axis. Polarized near-edge spectra show that the anisotropic bonding states of bromine are characterized by the Br-Br bond in the parallel direction and the Br-C bond in the perpendicular direction to the fibril axis, respectively. We propose that trans -(CHBr _y ) _x can be described in terms of highly oriented polybromine ions and brominated carbon chains.